This is another non-accounting voucher. However, it differs from the Memo voucher in that it is not a separate voucher type. You mark an existing voucher, (e.g., a payment voucher or a receipt voucher) as ‘Optional’. Press [CTRL]+[L] or click on the button. This button toggles with ‘Regular’. By marking it ‘Optional’ the voucher does not get posted anywhere but remains in the have the Optional Register.
You can make a voucher type default to ‘Optional’. This is done during creation of a new voucher type or alteration of an existing voucher type. (Gateway of Tally.ERP > Accounts Info > Voucher Types).